Note: For the non-eutrophicated and intermediate eutrophicated areas, the average annual biomass (gm~2) of each is given. For the eutrophicated area, the average biomass (gm~2) of each group before and after the algae crash is given.

Note: For the non-eutrophicated and intermediate eutrophicated areas, the average annual biomass (gm~2) of each is given. For the eutrophicated area, the average biomass (gm~2) of each group before and after the algae crash is given.

Specific eco-exergy was found to be consistently higher in the Z. noltii community than in the eutrophicated areas until late spring when the picture changed completely and values became higher in the eutrophicated areas. This was due to a macroalgae crash in the eutrophicated areas, which determined not only a drastic reduction of the total biomass but also a change from a primary production-based situation toward a detritus-based food web. Therefore, since total biomass values after late spring consisted basically of animals (consumers), primarily deposit feeders, and detritus feeders (e.g., annelid worms and crustaceans), it is clear that the abrupt increase of specific eco-exergy in the eutrophicated areas after the algae crash do not reflect any augmentation of the structural complexity of the community, but simply the different quality of the biomass involved in the calculations.

Regarding the Zostera community (data after 6 July), accounting for the primary producers and the consumers, specific exergy is lower than in the eutrophicated areas. But if we account only for the consumers, it is higher, following the same pattern from before the algae crash. Hence, specific eco-exergy may shift very drastically as a function of yearly dynamics (like in communities dominated by r-strategists), providing a spatial and temporal picture that may not be related with the long-term evolution and integrity of the system. With regard to biodiversity, the variation of species richness and of heterogeneity (species richness + evenness) along the gradient of eutrophication provided quite different pictures. Through time species richness was consistently higher in Zostera community, decreasing along the gradient of eutrophication. On the contrary, heterogeneity was always higher in the eutrophicated areas, except for the decrease observed in the most heavily eutrophicated area after an algae crash.

The observed spatial variation of heterogeneity is due to the fact that the Shannon-Wiener's index integrates two components: the number of species (species richness) and their relative abundance (evenness). Therefore, although species richness decreased as a function of increasing eutrophication, as we expected, the dominance of a few species (e.g., Hydrobia ulvae, a detritus feeder and epiphytic grazer gastropod, and Cerastoderma edule, a filter feeder bivalve) in the Zostera community, probably due to the abundance of nutritional resources, decreased species evenness, and consequently heterogeneity values. In this case, lower values of heterogeneity must be interpreted as expressing higher biological activity, and not as a result of environmental stress (Legendre and Legendre, 1984).

Taking into account the yearly data series for each site along the eutrophication gradient (non-eutrophicated, intermediate eutrophicated, eutrophicated), eco-exergy, and specific eco-exergy were significantly correlated (p < 0.05) providing a similar picture from the system. Values were consistently higher and more stable in the non-eutrophicated area. The comparison of yearly data series (t-test, p < 0.05) showed that using eco-exergy values it was possible to distinguish between the three situations considered, even though differences between the intermediate and eutrophicated areas were not significant, which suggests that eco-exergy, an extensive function, might be more sensitive to detect subtle differences.

Species richness and eco-exergy were significantly correlated (p < 0.05), following a similar pattern, both decreasing from non-eutrophicated to eutrophicated areas

(Figure 9.14B). On the contrary, heterogeneity and eco-exergy appeared negatively correlated (although not significantly), providing a totally distinct picture of the benthic communities along the eutrophication gradient (Figure 9.14A). This obviously resulted from the properties of the heterogeneity measure, as explained above.

Similar results were obtained comparing the patterns of variation of species richness, heterogeneity, and specific eco-exergy. Species richness and specific eco-exergy appeared clearly positively correlated (p < 0.05) (Figure 9.14B), while the patterns of variation of heterogeneity and specific eco-exergy showed to be distinct (Figure 9.14A). Moreover, from the comparison of yearly data series (i-test, p < 0.05), heterogeneity values were not significantly different in the intermediately eutrophicated and eutrophicated areas, and therefore did not permit to discriminate relatively subtle differences.

The hypothesis that eco-exergy and biodiversity would follow the same trends in space and time was validated with regard to species richness, but not for heterogeneity. Actually, eco-exergy, specific eco-exergy, and species richness responded as hypothesized, decreasing from non-eutrophicated to eutrophicated areas, but heterogeneity responded in the opposite way, showing the lowest values in the non-eutrophicated area.

Their range of variation (eco-exergy and specific eco-exergy) through time was smaller in the non-eutrophic area, expressing a more stable situation, while the magnitude of the variations was stronger in the other two areas, but especially in the intermediate eutrophic area (Marques et al., 2003). On the other hand, both eco-exergy and species richness were able to grade situations presenting relatively subtle differences, but specific eco-exergy and heterogeneity appeared to be less sensitive. Although biodiversity may be considered as an important property of ecosystem structure, the relative subjectivity of its measurements and their interpretation constitutes an obvious problem.

The spatial variation of species richness was significant; biodiversity may be seen as the full range of biological diversity from intraspecific genetic variation to the species richness, connectivity, and spatial arrangement of entire ecosystems at a landscape-level scale (Solbrig, 1991). If we accept this biodiversity concept, then eco-exergy, as system-oriented characteristic and as ecological indicator of ecosystem integrity, may encompass biodiversity.

Moreover, eco-exergy implies the existence of the transport information through scales, from the genetic to the ecosystem level, accounting not only for the biological diversity, but also for the evolutionary complexity of organisms, and ecosystem-emergent properties arising from self-organization.

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